I'm not sure, why is this site 18+ ?
Is there something that might be inappropriate for young people ? Even < 15 might be interested in these things and most interest about sex starts at very young age (about 12+).
So, this is on the meta for the site, but I'll copy and paste it over here since not everyone can see it yet.
It is a cautious restriction and simply a prudent way to start out until we get a better feel for where this site is going. Laws tend to get a more stringent when you start mixing minors and "adult material" and we wanted the flexibility to develop this site as we see fit without creating an administrative nightmare.
It may or may not turn out to be an issue. I don't pretend to be an expert on such legal questions. If our restrictions prove to be too stringent, it is easier to lower the bar later than to start booting well-meaning people off of the site.
This was posted by a moderator on the meta. As it states, this restriction at the moment is more to do with precaution than anything else.